For the purpose of continuously electro-tinplating a metallic material such as a metal strip or a metal wire, the following methods are known:
(1) Method using a soluble anode:
This method comprises the steps of: using an acidic electro-tinplating solution containing phenolsulfonic acid or a salt thereof and tin ion, using a soluble anode comprising metallic tin, and causing a DC electric current to flow between the soluble anode and a metallic material to be tinplated, thereby forming a tinplating layer on the surface of the metallic material.
(2) Method using an insoluble anode:
This method comprises the steps of: using an acidic electro-tinplating solution containing phenolsulfonic acid or a salt thereof and tin ion, using an insoluble anode comprising a titanium plate, the surface of which is platinum-plated, and causing a DC electric current to flow between the insoluble anode and a metallic material to be tinplated, while replenishing the acidic electro-tinplating solution with tin ion, thereby forming a tinplating layer on the surface of the metallic material.
The method using a soluble anode comprising metallic tin mentioned under (1) above is advantageous in that dissolution of the soluble anode permits automatic replenishment of the acidic electro-tinplating solution with tin ion. This method involves however the following problems:
(a) Since the soluble anode has a higher dissolution efficiency into the electro-tinplating solution than an electrodeposition efficiency of tin to the metallic material, the electro-tinplating solution would contain an excessive amount of tin ion. As a result, the thickness of the tinplating layer formed on the surface of the metallic material is apt to be non-uniform.
(b) Along with the progress of dissolution of the soluble anode, the distance between the soluble anode and the metallic material changes. In order to keep a constant distance between the soluble anode and the metallic material, therefore, it is necessary to move the soluble anode toward the metallic material.
(c) The soluble anode after dissolution over a certain limit must be replaced with a new one. The necessity of frequent replacement of the soluble anode reduces the operating efficiency of tinplating.
The method using an insoluble anode comprising a titanium plate, the surface of which is platinumplated, mentioned under (2) above is advantageous in that the electro-tinplating solution never contains an excessive amount of tin ion since the acidic electro-tinplating solution can be replenished with tin ion in an amount corresponding to the electrodeposition efficiency of tin to the metallic material. Furthermore, since the insoluble anode is hardly dissolved, a constant distance can always be kept between the insoluble anode and the metallic material, and the necessity of replacement of the insoluble anode is remarkably reduced. According to this method, therefore, the problems encountered in the above-mentioned method using a soluble anode are solved, and it is possible to efficiently form a tinplating layer having a uniform thickness on the surface of the metallic material. This method has however the following problems:
(a) Anodic oxidation of phenolsulfonic acid or a salt thereof on the surface of the insoluble anode in the acidic electro-tinplating solution, causes the production of denaturations of phenolsulfonic acid or the salt thereof. The thus produced denaturations aggregate in the tinplating solution, and the aggregated denaturations adhere to the tinplating layer formed on the surface of the metallic material, thus causing deterioration of the quality of the tinplating layer.
(b) Oxygen gas produced on the surface of the insoluble anode during electrolysis causes oxidation of tin ion contained in the acidic electro-tinplating solution, to produce a large quantity of sludge in the tinplating solution. The thus produced sludge adheres to the tinplating layer formed on the surface of the metallic material, thus causing deterioration of the quality of the tinplating layer.
As a means to solve the problem described under (a) above, there is known the following additive for dispersing the denaturations of phenolsulfonic acid or the salt thereof, produced in the acidic electro-tinplating solution, into the tinplating solution so as to prevent the deterioration of the quality of the tinplating layer:
An additive for an acidic electro-tinplating solution, as represented by the following general formula (hereinafter referred to as the "prior art"), disclosed in Japanese Patent Provisional Publication No. 61-73,896 dated Apr. 16, 1986: ##STR1##
The above-mentioned prior art has the following problems:
(1) There is available only a limited effect of dispersing the denaturations of phenolsulfonic acid or the salt thereof, produced in the acidic electro-tinplating solution, into the tinplating solution so as to prevent the deterioration of the quality of the tinplating layer.
(2) It is impossible to inhibit the production of sludge caused by the oxidation of tin ion in the acidic electro-tinplating solution.
Under such circumstances, there is a strong demand for the development of a method for stably forming a high-quality tinplating layer on the surface of a metallic material, which permits, when using an acidic electro-tinplating solution containing phenolsulfonic acid or a salt thereof and tin ion, using an insoluble anode, and causing a DC electric current to flow between the insoluble anode and the metallic material, while replenishing the acidic electro-tinplating solution with tin ion, thereby forming a tinplating layer on the surface of the metallic material, prevention of the production of denaturations of phenolsulfonic acid or the salt thereof in the acidic electro-tinplating solution, and furthermore, inhibition of the production of sludge caused by the oxidation of tin ion in the acidic electro-tinplating solution, but such a method has not as yet been proposed.